Sensing of viral infections in C. elegans - SUMMARY RNA viruses pose a threat to organisms ranging from bacteria to humans, and thus all hosts need to recognize and defend themselves from these foreign invaders. While some principles of RNA virus recognition have been defined, primarily in mammalian systems, there remains a significant gap in knowledge as to how different hosts recognize the presence of an RNA virus. Much less is known about how simple metazoans, such as the model organism C. elegans, recognize virus infection. Studies in C. elegans have led to the discovery of multiple fundamental paradigms in biology, such as identification of caspase cell death pathways and RNA interference (RNAi). The discovery over a decade ago of Orsay virus (ORV), a positive strand RNA virus that is the first and only known natural virus of C. elegans to date, opened the door to virus-host interaction studies in this simple, multicellular invertebrate model system. Defining the host genes required for recognizing an RNA virus and the viral derived molecules that are sensed by C. elegans will provide novel insights into the evolution of self-versus-nonself discrimination. These studies could also lead to identification of novel viral recognition and signaling pathways that are broadly conserved. In mammals a family of cytoplasmic receptors that includes RIG- I and MDA5 senses the presence of intracellular RNA virus derived products. They then signal through additional proteins, such as MAVS, to activate transcription factors, including IRF3, IRF7, and NF-kB to induce an antiviral transcriptional response. C. elegans encodes DRH-1, a dicer related helicase that is homologous to RIG-I and MDA5, that is required for activation of the C. elegans transcriptional response, termed the intracellular pathogen response (IPR), to ORV infection. However, since C. elegans does not possess orthologs of MAVS, IRF3, IRF7 or NF-kB, it is not clear how the sensing of an RNA virus is transduced in C. elegans and what genes are involved. The viral derived molecule(s) that are recognized in C. elegans is also poorly defined. RIG-I binds viral dsRNA with a 5' triphosphate while MDA5 binds longer dsRNAs independent of the 5' terminus. In C. elegans, replication of ORV from a plasmid-based replicon system is sufficient to activate the IPR implicating an ORV replication product as the trigger. In preliminary data, we demonstrated that non-replicating ORV dsRNA produced by the endogenous C. elegans transcription machinery is sufficient to activate the IPR in vivo. This implies that viral dsRNA with 5' cap structure can be a trigger, providing the first evidence that DRH-1 maybe more similar to MDA5 than RIG-1. Thus, studies of DRH-1 may provide novel insights into the evolution and function of MDA5-like sensing proteins. Here, we will (1) define the host factors that recognize viral infection and are responsible for transducing that signal into a transcriptional response, and (2) determine the precise nature and characteristics of the viral ligand(s) that is recognized by C. elegans.
